Hybrid Electric Vehicle Basics

This diesel hybrid electric bus operated by the Metropolitan Transit Authority, New York City Transit, was part of a test study that recently investigated the fuel efficiency and reliability of these buses.Credit: Leslie Eudy

Today's hybrid electric vehicles (HEVs) range from small passenger cars to sport utility vehicles (SUVs) and large trucks. Though they often look just like conventional vehicles, HEVs usually include an electric motor as well as a small internal combustion engine (ICE). This combination provides greater fuel economy and fewer emissions than most conventional ICE vehicles do.

HEVs are powered by two energy sources: an energy conversion unit, such as a combustion engine or fuel cell, and an energy storage device, such as a battery pack or ultracapacitor. The energy conversion unit may be powered with gasoline or an alternative fuel, such as an ethanol-petroleum blend.

The electric battery pack is recharged by an onboard generator or a regenerative brake. Regenerative braking captures some of the kinetic energy that would otherwise be lost as heat during braking.

There are two types of HEV designs:

Parallel design—In this design, the energy conversion unit and an electric propulsion system are connected directly to the vehicle's wheels. The vehicle uses the primary engine for highway driving, and the electric motor provides additional power for acceleration, climbing hills, and other high-demand activities.

Series design—In this design, the primary engine is connected to a generator that produces electricity. The electricity charges the batteries, which drive an electric motor that powers the wheels.